This invention relates generally to apparatus for discriminating among sounds and for communicating the existence of a predetermined sound and more particularly, but not by way of limitation, to apparatus for warning of the presence of an emergency signal having a predetermined frequency as emitted by an emergency vehicle or the like.
A person driving a vehicle should be constantly aware of ambient sounds because the vehicle must be operated to yield the right of way to an emergency vehicle which is emitting an emergency signal (e.g., an ambulance). Maintaining the necessary constant awareness is often difficult for the driver because, for example, he or she may have a physical hearing impairment or he or she may have the vehicle windows rolled up and the air conditioner and/or radio turned on. Emergency signals may also not be detected by the driver if the vehicle has a high degree of noise associated with it as may be the case in a school bus carrying several children. To relieve the driver of the difficult task of continuously listening for particular ambient sounds, there is the need for a device which detects sounds external to the compartment in which the vehicle driver is riding, which discriminates among all detected ambient sounds to select an emergency signal emitted by an emergency vehicle, and which communicates this detection and discrimination to the driver.
It is known that sirens, such as may be on ambulances, and car horns emit attention-attracting sounds when they are operated. These sounds have frequencies primarily, if not entirely, within the frequency band from approximately 2 kHz to approximately 4 kHz because research has shown that sounds in this range are the most attention-attracting to humans. Because the emergency signals are within this range, an apparatus which warns the driver of the existence of such signals must provide means for detecting signals within this range.
Although it is known that warning signals provided by emergency vehicles have frequencies within the aforementioned range, appropriate detection of the warning signals cannot be made by merely monitoring this one frequency range because ambient noise can create false warning signals. For example, it is known that the relative wind movement between the vehicle in which the driver to be warned is riding and the atmosphere around the vehicle produces signals varying with the speed of the vehicle. It is known that this wind noise produces high frequency signals which can be used to reduce or prevent the detection of false warning signals. This is illustrated in U.S. Pat. No. 4,158,190 in the name of Stefanov. The described Stefanov device uses a high-pass filter to detect erratic, random signals and to inhibit the driver notification means when such signals are detected so that the driver is not misinformed by a false signal.
Although it is known that wind produces high frequency signals which can be used in the aforementioned manner, we have found that the wind noise factor is not as much a high frequency shift with car velocity as it is an amplitude increase over a band of frequencies. Specifically, the majority of energy in this noise is contained in the range of frequencies below 1 kHz.
Utilizing the energy of the wind noise signals in this relatively low frequency band to set a threshold against which the warning signals, having relatively higher, predetermined frequencies, detected by the device are compared enables the sensitivity of the device to be increased by a significant factor over a device using a high-pass filter to measure the energy in the wind noise at frequencies above the frequencies of the true warning signal and to set a threshold based thereon. This low frequency energy utilization gives the driver greater protection at low driving speeds because the threshold is relatively low resulting from the lesser relative wind speed. Although adequate protection is also provided for high speed highway driving, the greater low speed protection is most important because most accidents involving emergency vehicles occur at in-town speeds, such as below 40 mph.
Utilizing the low frequency energy is also advantageous because it minimizes device performance variations resulting from differences in types of microphones which may be used. This variation minimization results because the low frequency threshold detector effectively utilizes the ratio of band-pass energy to lowpass energy.